1. Field of the Invention
The present invention relates to an acoustic apparatus in which a vibrator is arranged in a Helmholtz's resonator having an open duct port, and is driven to radiate a resonant acoustic wave and, more particularly, to an acoustic apparatus in which an unnecessary resonant sound other than a Helmholtz's resonant sound produced when the Helmholtz's resonator is driven is eliminated, thereby removing noise in a radiated acoustic wave and improving distortion characteristics.
2. Description of the Prior Art
As an acoustic apparatus solely utilizing a Helmholtz's resonance, a phase-inversion (bass-reflex) speaker system is known. FIGS. 15A and 15B are respectively a perspective view and a sectional view showing an arrangement of the bass-reflex speaker system. In the speaker system shown in FIGS. 15A and 15B, a hole is formed in the front surface of an enclosure 6, a vibrator consisting of a diaphragm 2 and a dynamic speaker 3 is mounted in the hole, and an open duct port 8 having a sound path 7 is formed therebelow. In the bass-reflex speaker system according to the conventional basic design, a resonance frequency f.sub.OP defined by an air spring of the enclosure 6 and an air mass in the sound path 7 is set to be lower than a lowest resonance frequency f.sub.0 of the vibrator (speaker) when the vibrator is assembled in the bass-reflex enclosure. At a frequency higher than the resonance frequency defined by the air spring and the air mass, the phase of sound pressure from the rear surface of the diaphragm 2 is inverted at the sound path 7. Consequently, in front of the enclosure 6, a sound directly radiated from the front surface of the diaphragm 2 is in phase with a sound from the open duct port, thus increasing the sound pressure. As a result, according to an optimally designed bass-reflex speaker system, the frequency characteristics of the output sound pressure can be expanded to the resonance frequency f.sub.0 of the vibrator or less. As indicated by an alternate long and two short dashed curve in FIG. 16, a uniform reproduction range can be widened as compared to an infinite plane baffle or closed baffle.
However, in the bass-reflex speaker system, open duct resonance occurs at the open duct port portion, and the resonant sound is radiated as noise or a distortion component of an acoustic wave.
In order to eliminate such distortion or noise, another acoustic apparatus wherein a small-diameter portion is formed in the central portion of a port to eliminate port resonance has been proposed (Japanese Utility Model Publication No. Sho 54-35068). However, in this case, as the diameter of the small-diameter portion is decreased to enhance a filter effect, an acoustic resistance of the port is increased, and the Q value of the Helmholtz's resonance is decreased. As a result, the behavior of the speaker system approximates an operation in a closed space, and its frequency characteristics approximate those indicated by an alternate long and short dashed curve in FIG. 16. Therefore, bass-sound radiation power is decreased.